Pneumonia is an inflammation of the lung parenchyma (Fig 93-4). Consolidation of the lung tissue may be identified by physical examination and chest x-ray. From an anatomical point of view, lobar pneumonia denotes an alveolar process involving an entire lobe of the lung while bronchopneumonia describes an alveolar process occurring in a distribution that is patchy without filling an entire lobe. Numerous factors, including environmental contaminants and autoimmune diseases, as well as infection, may cause pneumonia. The various infectious agents that cause pneumonia are categorized in many ways for purposes of laboratory testing, epidemiologic study and choice of therapy. Pneumonias occurring in usually healthy persons not confined to an institution are classified as community-acquired pneumonias. Infections arise while a patient is hospitalized or living in an institution such as a nursing home are called hospital-acquired or nosocomial pneumonias. Etiologic pathogens associated with community-acquired and hospital-acquired pneumonias are somewhat different. However, many organisms can cause both types of infections.